Gel electrolyte, process for producing the same, and use thereof

ABSTRACT

A uniform gel electrolyte having high durability and use thereof, in particular, batteries or capacitors using such a gel electrolyte. The gel electrolyte includes a gel composition containing an electrolyte salt, a solvent for the electrolyte salt, and a polymer matrix, wherein the polymer matrix comprises a crosslinked polymer prepared by polymerizing a bifunctional (meth)acrylate represented by the following formula (I): 
                         
wherein R represents a divalent organic group, and R 1  represents a hydrogen atom or a methyl group.

This is a divisional of application Ser. No. 10/383,071 filed Mar. 7,2003 now U.S. Pat. No. 7,097,940. The entire disclosure of the priorapplication, application Ser. No. 10/383,071, is hereby incorporated byreference.

FIELD OF THE INVENTION

The present invention relates to a gel electrolyte, and particularly, toa gel electrolyte that can be suitably used in, for example, batteriesor capacitors.

DESCRIPTION OF THE RELATED ART

A solid electrolyte as referred to herein means a substance that is in asolid state and has a high ionic conductivity. Especially, in recentyears, polymer solid electrolytes in which a polymer substance is usedas a solid are watched as an electrolyte for next-generation lithiumsecondary batteries, and their researches are worldwide promoted.

As compared with the conventional electrolytic solutions, such polymersolid electrolytes are free from a fear of liquid leakage and have alarge degree of freedom of the shape such that they can be processedinto a thin film. However, the conventional polymer solid electrolytesinvolve a problem such that the conductivity is extremely low ascompared with liquid electrolytes, i.e., electrolytic solutions. Forexample, there have hitherto been known non-aqueous polymer solidelectrolytes comprising a composite of a chain polymer (such aspolyethylene glycol and polypropylene glycol) or a comb-like polymer(such as polyphosphazene) and an electrolyte salt. However, there hashitherto been not found any solid electrolyte having a conductivityexceeding 10⁻³ S/cm at room temperature.

Then, in recent years, various non-aqueous gel electrolytes areresearched for making them for practical use. According to theseresearches, there are proposed ones having a conductivity of 10⁻³ S/cmor more at room temperature and close to electrolytic solutions. Suchgel-like electrolytes comprise an electrolyte salt dissolved in a gelformed from a polymer material and a non-aqueous organic solvent and canbe, for example, obtained by holding an electrolytic solution in apolymer matrix.

Examples of these gel electrolytes are conventional gel electrolytescomprising an electrolytic solution carried in pores of a porousfluorocarbon polymer made of polyvinylidene fluoride or a vinylidenefluoride/hexafluoroporpylene copolymer as disclosed in, for example,JP-T-8-509100 and JP-T-9-500485 (the term “JP-T” as used herein means apublished Japanese translation of a PCT patent application).

However, in these gel electrolytes, it is impossible to make the amountof the electrolytic solution carried in the porous fluorocarbon polymerso high. Accordingly, for example, in the case where such a gelelectrolyte is used for a battery, sufficient battery characteristicscannot be obtained. In addition, since the electrolytic solution iscarried in a liquid state in the pores of the porous polymer, there is apossibility that it causes liquid leakage.

On the other hand, JP-A-8-298126 proposes gel electrolytes comprisingpolyethylene oxide or a polyether polymer analogous thereto as a polymermatrix and also proposes gel electrolytes comprising a polyether polymer(such as polyethylene oxide and polypropylene oxide) as a polymer matrixand γ-butyrolactone as an organic solvent. Further, JP-A-11-176452proposes gel electrolytes using a tetrafunctional terminalacryloyl-modified alkylene oxide polymer.

These polyether-based polymers give gel electrolytes that have a highcompatibility with electrolytic solutions, are homogeneous and have ahigh conductivity. However, in the case where lithiumhexafluorophosphate as generally used in lithium-ion batteries is usedas an electrolyte salt, the polymers cause decomposition, resulting in aproblem in durability.

SUMMARY OF THE INVENTION

The invention has been made to overcome the above-described problems ofthe conventional gel electrolytes.

Accordingly, an object of the invention is to provide a uniform gelelectrolyte having high durability and its use, and particularly, toprovide batteries or capacitors using such a gel electrolyte.

According to the invention, there is provided a gel electrolytecomprising a gel composition comprising an electrolyte salt, a solventfor the electrolyte salt, and a polymer matrix, wherein the polymermatrix comprises a crosslinked polymer prepared by polymerizing abifunctional (meth)acrylate represented by the following formula (I):

wherein R represents a divalent organic group, and R₁ represents ahydrogen atom or a methyl group.

Further, according to the invention, there is provided a process forproducing a gel electrolyte, which comprises heating or irradiating withactinic radiations a solution containing an electrolyte salt, a solventfor the electrolyte salt, and a bifunctional (meth)acrylate representedby the following formula (I):

wherein R represents a divalent organic group, and R₁ represents ahydrogen atom or a methyl group,to polymerize the bifunctional (meth)acrylate, thereby forming acrosslinked polymer and forming a gel in which the electrolyte salt andthe solvent are held in a matrix comprising the crosslinked polymer.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE is a cross-sectional view of a coin-type secondary batteryusing a gel electrolyte according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the gel electrolyte according to the invention, the polymer matrixcomprises a crosslinked polymer prepared by polymerizing thebifunctional (meth)acrylate represented by the above-described formula(I).

In the invention, the term “(meth)acrylate” means an acrylate or amethacrylate, and the term “(meth)acryloyl” means an acryloyl or amethacryloyl.

The bifunctional (meth)acrylate can be obtained by reacting a diaminerepresented by the following formula (IV):H₂N—R—NH₂   (IV)wherein R represents a divalent organic group, with2-(meth)acryloyloxyethyl isocyanate in a reaction solvent. Accordingly,in the bifunctional (meth)acrylate, the divalent organic grouprepresented by R is a residue of the diamine used as the startingmaterial for the production (that is, a group in which two amino groupsof the diamine have been eliminated).

Thus, according to the invention, although the divalent organic group ordiamine residue is not particularly limited, preferred examples include:

(a) an alkylene group,

(b) a divalent group represented by the following formula (II):

wherein R₂ represents an alkyl group, and R₃ and R₄ each independentlyrepresents an alkylene group,

(c) a xylylene group, and

(d) a divalent group represented by the following formula (III):

Of these organic groups, the alkylene group (a) is preferably a linearor branched alkylene group having 2 to 40 carbon atoms, such as anethylene group, a propylene group, a hexamethylene group, and adodecamethylene group. In the divalent group represented by the formula(II), the alkyl group represented by R₂ preferably has 1 to 10 carbonatoms; and the alkylene group represented by R₃ and R₄ preferably has 1to 10 carbon atoms. Accordingly, a preferred example of the divalentgroup represented by the formula (II) is one in which R₁ represents amethyl group, and R₂ and R₃ each represents a trimethylene group.

Examples of the xylylene group include an o-xylylene group, anm-xylylene group and a p-xylylene group, and an m-xylylene group isparticularly preferred. Examples of the divalent group represented bythe formula (III) include:

Of those, a diamine residue derived from 1,3-bis(aminomethyl)cyclohexaneis preferable.

The gel electrolyte according to the invention can be obtained by, forexample, dissolving the bifunctional (meth)acrylate in an electrolyticsolution comprising an electrolyte salt and a solvent for theelectrolyte salt preferably together with a polymerization initiator,and heating the resulting solution, thereby polymerizing (heatpolymerizing) the bifunctional (meth)acrylate and forming a crosslinkedpolymer as the polymer matrix. In other words, according to theinvention, the bifunctional (meth)acrylate is used as a gelling agent.

The polymerization initiator for polymerizing the bifunctional(meth)acrylate is not particularly limited, but examples thereof includebenzoyl peroxide and 2,2′-azobisisobutyronitrile. However, thepolymerization method of the bifunctional (meth)acrylate is not limitedto the heat polymerization, but the bifunctional (meth)acrylate may bephoto-polymerized upon irradiation with actinic radiations such asultraviolet ray and electron beams.

In the gel electrolyte according to the invention, a proportion of thecrosslinked polymer prepared by polymerizing the bifunctional(meth)acrylate is appropriately determined depending on the electrolytesalt and solvent used such that they form the gel electrolyte comprisingthe crosslinked polymer as the matrix. However, the proportion of thecrosslinked polymer is usually in the range of from 0.1 to 50% by weightof the gel electrolyte. Especially, in the case where the gelelectrolyte according to the invention is used as a gel electrolyte forbatteries or capacitors, when the proportion of the polymer matrix inthe gel electrolyte is large, the electric characteristics are lowered.Accordingly, the proportion of the crosslinked polymer is preferably inthe range of from 0.1 to 25% by weight, and more preferably from 0.1 to10% by weight, of the gel electrolyte.

Further, a proportion of the electrolyte salt in the gel electrolyteaccording to the invention is appropriately determined depending on not:only the electrolyte salt itself but also the solvent used. Theproportion of the electrolyte salt is usually in the range of from 1 to50% by weight in the obtained gel electrolyte.

The electrolyte salt that can be used in the invention is saltscomprising hydrogen, an alkali metal (such as lithium, sodium, andpotassium), an alkaline earth metal (such as calcium and strontium), ora tertiary or quaternary ammonium salt as a cationic component and aninorganic acid (such as hydrochloric acid, nitric acid, phosphoric acid,sulfuric acid, fluoroboric acid, hydrofluoric acid, hexafluorophosphoricacid, and perchloric acid) or an organic acid (such as organiccarboxylic acids, organic sulfonic acids, and fluorine-substitutedorganic sulfonic acids) as an anionic component. Especially, electrolytesalts comprising an alkali metal ion as a cationic component arepreferably used.

Specific examples of the electrolyte salts comprising such an alkalimetal ion as a cationic component include alkali metal perchlorates suchas lithium perchlorate, sodium perchlorate, and potassium perchlorate;alkali metal tetrafluoroborates such as lithium tetrafluoroborate,sodium tetrafluoroborate, and potassium tetrafluoroborate; alkali metalhexafluorophosphates such as lithium hexafluorophosphate and potassiumhexafluorophosphate; alkali metal trifluoroacetates such as lithiumtrifluoroacetate; and alkali metal trifluoromethanesulfonates such aslithium trifluoromethanesulfonate.

In addition, any solvent can be used as the solvent for the electrolytesalt in the invention so far as it can dissolve the electrolyte salttherein. Examples of non-aqueous solvents include cyclic esters such asethylene carbonate, propylene carbonate, butylene carbonate, andγ-butyrolactone; cyclic or chain ethers such as tetrahydrofuran anddimethoxyethane; and chain esters such as dimethyl carbonate, diethylcarbonate, and ethylmethyl carbonate. These solvents may be used aloneor as mixtures of two or more thereof.

In the case where the gel electrolyte according to the invention is usedfor batteries or capacitors, in order to prevent a short circuit betweenelectrodes, the gel electrolyte can be used as a film-like gelelectrolyte by appropriately carrying the same on a sheet-likesubstrate. When such a substrate is used, it is preferred to use aporous film as the sheet-like substrate not so as to obstruct themovement of ions between the electrodes.

That is, according to the invention, in the substrate porous film, thesolution containing the electrolyte salt, the solvent for theelectrolyte salt, and the bifunctional (meth)methacrylate represented bythe formula (I) is heated or irradiated with actinic radiations topolymerize the bifunctional (meth)acrylate, thereby forming thecrosslinked polymer and forming the gel in which the electrolyte saltand solvent are held in the matrix comprising the crosslinked polymer.There can be thus obtained the film-like gel electrolyte.

For example, the film-like gel electrolyte can be suitably used for theproduction of batteries or capacitors. For example, as a first method,electrodes and a substrate porous film are laminated or wound to form anelectrochemical element, which is then installed in a battery can alsofunctioning as an electrode can of the battery. Next, the solutioncontaining the electrolyte salt, the solvent for the electrolyte saltand the bifunctional (meth)acrylate, and preferably the polymerizationinitiator is poured into the battery can, and the substrate porous filmis impregnated with the solution. Thereafter, the bifunctional(meth)acrylate is polymerized upon heating, thereby forming thecrosslinked polymer and forming a homogeneous gel electrolyte comprisingthe crosslinked polymer as the polymer matrix. There can be thusobtained a battery containing the film-like gel electrolyte.

As a second method, the bifunctional (meth)acrylate is previouslycarried on the substrate porous film, and the resulting porous film andelectrodes are laminated or wound to form an electrochemical element,which is then installed in a battery can also functioning as anelectrode can of the battery. Next, an electrolytic solution comprisingthe electrolyte salt and the solvent for the electrolyte salt, andpreferably the polymerization initiator is poured into the battery can,and the substrate porous film is impregnated with the electrolyticsolution to dissolve the bifunctional (meth)acrylate carried on thesubstrate porous film in the electrolytic solution. Thereafter, thebifunctional (meth)acrylate is polymerized upon heating, thereby formingthe crosslinked polymer and forming a uniform gel electrolyte comprisingthe crosslinked polymer as the polymer matrix. There can be thusobtained a battery containing the film-like gel electrolyte.

As a third method, the bifunctional (meth)acrylate and thepolymerization initiator are previously carried on the substrate porousfilm, and the resulting porous film and electrodes are laminated orwound to form an electrochemical element, which is then installed in abattery can also functioning as an electrode can of the battery. Next,an electrolytic solution comprising the electrolyte salt and the solventfor the electrolyte salt is poured into the battery can, and thesubstrate porous film is impregnated with the electrolytic solution todissolve the bifunctional (meth)acrylate and the polymerizationinitiator carried on the substrate porous film in the electrolyticsolution. Thereafter, the bifunctional (meth)acrylate is polymerizedupon heating, thereby forming the crosslinked polymer and forming ahomogeneous gel electrolyte comprising the crosslinked polymer as thepolymer matrix. There can be thus obtained a battery containing thefilm-like gel electrolyte.

In addition, as another method, the bifunctional (meth)acrylate and thepolymerization initiator are previously dissolved in an electrolyticsolution comprising the electrolyte salt and the solvent for theelectrolyte salt to prepare a solution. After impregnating the substrateporous film with the solution and impregnating electrodes (positiveelectrode and negative electrode) with the electrolytic solution, theresulting substrate porous film and electrodes are installed in abattery can, and a laminate comprising, for example, negativeelectrode/substrate porous film/positive electrode is formed within thebattery can, to prepare an intermediate of a battery. Thereafter, theintermediate is heated to polymerize the bifunctional (meth)acrylatecontained in the substrate porous film, thereby forming the crosslinkedpolymer and forming a uniform gel electrolyte comprising the crosslinkedpolymer as the polymer matrix. There can be thus obtained a batterycontaining the film-like gel electrolyte. It will be easily understoodthat capacitors can be obtained similarly by the above-describedmethods.

The FIGURE is a cross-sectional view of a coin-type lithium secondarybattery using the gel electrolyte. In this lithium secondary battery, apositive electrode can 1 also functioning as a positive electrodeterminal is made of, for example, a nickel-plated stainless steel sheetand is combined with a negative electrode can 3 also functioning as anegative electrode terminal, which is insulated from the positiveelectrode can 1 via an insulator 2, to configure a battery can (vessel).The negative electrode can 3 is also made of, for example, anickel-plated stainless steel sheet.

Inside the thus formed battery can, a positive electrode 4 is arrangedwhile contacting with the positive electrode can 1 via a positiveelectrode collector 5. For example, the positive electrode 4 can beobtained by mixing a positive electrode active material (such as alithium-manganese composite oxide) and a conductive material (such asgraphite) with a binder resin (such as polyethylene, polypropylene, andpolytetrafluoroethylene) and pressure molding the mixture. Similarly, anegative electrode 6 is arranged while contacting with the negativeelectrode can 3 via a negative electrode collector 7. For example, thenegative electrode 6 is made of a lithium sheet. A film-like gelelectrolyte 8 according to the invention is arranged between thepositive electrode 4 and the negative electrode 6 to configure abattery. According to the thus configured battery, an electric energycan be taken out while functioning the positive electrode can 1 and thenegative electrode can 3 as terminals.

The invention will be described below in more detail with reference tothe Examples together with the Referential Examples and ComparativeExamples, but it should not be construed that the invention is limitedthereto.

REFERENCE EXAMPLE 1

25.2 g of ethylenediamine was dissolved in 700 ml of toluene, and theresulting solution was heated to remove moisture as an azeotrope withtoluene. The resulting solution was cooled to room temperature, and 4.8g of 2-methacryloyloxyethyl isocyanate was then added dropwise to thesolution over 10 minutes with stirring. The resulting mixture wasreacted at room temperature for an additional 2 hours with stirring. Aprecipitated reaction product was collected by filtration andcrystallized with a mixed solvent of methanol and toluene to obtain abifunctional methacrylate represented by the following formula (1).Molecular weight by mass analysis (ESI method): (M+H)⁺=371Proton NMR spectrum (400 MHz, solvent: deutero acetic acid, δ (ppm)):

TABLE 1 (1)

Assignment δ (ppm) Signal Number of protons 1 6.00 m 2 2 5.48 m 2 3 4.08t 4 4 3.34 t 4 5 3.15 s 4 6 1.79 m 6¹³C-NMR spectrum (100 MHz, solvent: deutero acetic acid, δ (ppm)):

TABLE 2 (1)

Number of carbon Assignment δ (ppm) Signal atoms 1 169.4 s 2 2 162.0 s 23 138.0 s 2 4 127.4 t 2 5 65.6 t 2 6 41.9 t 2 7 40.6 t 2 8 19.1 q 2

REFERENCE EXAMPLE 2

30.3 g of hexamethylenediamine was dissolved in 1 liter of toluene, andthe resulting solution was heated to remove moisture as an azeotropewith toluene. The resulting solution was cooled to room temperature, and84.9 g of 2-methacyloxyethyl isocyanate was then added dropwise to thesolution over 10 minutes with stirring. Thereafter, the mixture wasreacted at room temperature for an additional 2 hours with stirring. Aprecipitated reaction product was collected by filtration andcrystallized with a mixed solvent of methanol and toluene to obtain abifunctional methacrylate represented by the following formula (2).Molecular weight by mass analysis (ESI method): (M+H)⁺=427Proton NMR spectrum (400 MHz, solvent: deutero acetic acid, δ (ppm)):

TABLE 3 (2)

Assignment δ (ppm) Signal Number of protons 1 6.00 m 2 2 5.49 m 2 3 4.09t 4 4 3.36 t 4 5 3.00 t 4 6 1.79 s 6 7 1.36 m 4 8 1.19 m 4¹³C-NMR spectrum (100 MHz, solvent: deutero acetic acid, δ (ppm)):

TABLE 4 (2)

Number of carbon Assignment δ (ppm) Signal atoms 1 169.6 s 2 2 161.8 s 23 138.0 s 2 4 127.4 t 2 5 65.7 t 2 6 41.9 t 2 7 40.9 t 2 8 31.2 t 2 927.9 t 2 10 19.0 q 2

REFERENCE EXAMPLE 3

34.3 g of methyliminobis(propylamine) was dissolved in 200 ml oftoluene, and the resulting solution was heated to remove moisture as anazeotrope with toluene. The resulting solution was cooled to roomtemperature, and 15.7 g of 2-methacryloxyethyl isocyanate was then addeddropwise to the solution over 10 minutes with stirring. Thereafter, theresulting mixture was reacted at room temperature for an additional 2hours with stirring. A precipitated reaction product was collected byfiltration and crystallized with a mixed solvent of methanol and tolueneto obtain a bifunctional methacrylate represented by the followingformula (3).Molecular weight by mass analysis (ESI method): (M+H)⁺=456Proton NMR spectrum (400 MHz, solvent: deutero acetic acid, δ (ppm)):

TABLE 5 (3)

Assignment δ (ppm) Signal Number of protons 1 6.00 m 2 2 5.49 m 2 3 4.09t 4 4 3.34 t 4 5 3.16 t 4 6 3.02 m 4 7 2.74 s 3 8 1.85 m 4 9 1.81 s 6¹³C-NMR spectrum (100 MHz, solvent: deutero acetic acid, δ (ppm)):

TABLE 6 (3)

Number of carbon Assignment δ (ppm) Signal atoms 1 169.3 s 2 2 161.9 s 23 137.9 s 2 4 127.3 t 2 5 65.4 t 2 6 55.2 t 2 7 40.9 q 1 8 40.8 t 2 938.2 t 2 10 26.4 t 2 11 19.0 q 2

REFERENCE EXAMPLE 4

17.1 g of m-xylylenediamine was dissolved in 1 liter of toluene, and theresulting solution was heated to remove moisture as an azeotrope withtoluene. The resulting solution was cooled to room temperature, and 40.8g of 2-methacryloyloxyethyl isocyanate was then added dropwise to thesolution over 10 minutes with stirring. Thereafter, the mixture wasreacted at room temperature for an additional 3 hours with stirring. Aprecipitated reaction product was collected by filtration andcrystallized with a mixed solvent of methanol and toluene to obtain abifunctional methacrylate represented by the following formula (4).Molecular weight by mass analysis (ESI method): (M+H)⁺=447Proton NMR spectrum (400 MHz, solvent: deutero acetic acid, δ (ppm)):

TABLE 7 (4)

Assignment δ (ppm) Signal Number of protons 1 7.14 to m 4 7.03 2 5.97 m2 3 5.47 m 2 4 4.22 s 4 5 4.09 t 4 6 3.38 t 4 7 1.77 s 6¹³C-NMR spectrum (100 MHz, solvent: deutero acetic acid, δ (ppm)):

TABLE 8 (4)

Number of carbon Assignment δ (ppm) Signal atoms 1 169.6 s 2 2 161.7 s 23 141.2 s 2 4 138.0 s 2 5 130.4 d 1 6 127.8 d 3 7 127.5 t 2 8 65.8 t 2 945.5 t 2 10 40.9 t 2 11 19.1 q 2

REFERENCE EXAMPLE 5

9.0 g of 1,3-bis(aminomethyl)cyclohexane was dissolved in 1 liter oftoluene, and the resulting solution was heated to remove moisture as anazeotrope with toluene. The resulting solution was cooled to roomtemperature, and 25.8 g of 2-methacryloxyethyl isocyanate was then addeddropwise to the solution over 30 minutes with stirring. Thereafter, themixture was reacted at room temperature for an additional 3 hours withstirring. A precipitated reaction product was collected by filtrationand crystallized with a mixed solvent of methanol and toluene to obtaina bifunctional methacrylate represented by the following formula (5).Molecular weight by mass analysis (ESI method): (M+H)⁺=453Proton NMR spectrum (400 MHz, solvent: deutero acetic acid, δ (ppm)):

TABLE 9 (5)

Assignment δ (ppm) Signal Number of protons 1 6.00 m 2 2 5.49 m 2 3 4.09t 4 4 3.35 t 4 5 2.87 m 4 6 1.79 s 6 7 1.61 m 4 8 1.32 m 2 9 1.12 m 1 100.69 m 2 11 0.43 m 1¹³C-NMR spectrum (100 MHz, solvent: deutero acetic acid, δ (ppm)):

TABLE 10 (5)

Number of carbon Assignment δ (ppm) Signal atoms 1 169.5 s 2 2 161.9 s 23 137.9 s 2 4 127.5 t 2 5 65.7 t 2 6 48.2 t 2 7 40.8 t 2 8 39.6 d 2 936.3 t 1 10 32.0 t 2 11 26.9 t 1 12 19.1 q 2

REFERENCE EXAMPLE 6

10.5 g of 1,12-dodecanediamine was dissolved in 500 ml of toluene, andthe resulting solution was heated to remove moisture as an azeotropewith toluene. The resulting solution was cooled to room temperature, and17.1 g of 2-methacryloxyethyl isocyanate was then added dropwise to thesolution over 10 minutes with stirring. Thereafter, the mixture wasreacted at room temperature for an additional 3 hours with stirring. Aprecipitated reaction product was collected by filtration andcrystallized with a mixed solvent of methanol and toluene to obtain abifunctional methacrylate represented by the following formula (6).Molecular weight by mass analysis (ESI method): (M+H)⁺=511Proton NMR spectrum (400 MHz, solvent: deutero dimethyl sulfoxide, δ(ppm)):

TABLE 11 (6)

Assignment δ (ppm) Signal Number of protons 1 6.05 s 2 2 5.80 m 4 3 5.65m 2 4 4.07 m 4 5 3.28 q 4 6 2.97 q 4 7 1.90 s 6 8 1.36 m 4 9 1.25 m 16¹³C-NMR spectrum (100 MHz, solvent: deutero dimethyl sulfoxide, δ(ppm)):

TABLE 12 (6)

Number of carbon Assignment δ (ppm) Signal atoms 1 166.5 s 2 2 158.3 s 23 136.0 s 2 4 125.3 t 2 5 64.1 t 2 6 39.2 t 2 7 38.4 t 2 8 29.9 t 2 928.9 to 28.7 t 6 10 26.3 t 2 11 17.8 q 2

REFERENCE EXAMPLE 7

14.9 g of 1,2-propanediamine was dissolved in 500 ml of toluene, and theresulting solution was heated to remove moisture as an azeotrope withtoluene. The resulting solution was cooled to room temperature, and 65.4g of 2-methacryloxyethyl isocyanate was then added dropwise to thesolution over 10 minutes with stirring. Thereafter, the mixture wasreacted at room temperature for an additional 3 hours with stirring. Aprecipitated reaction product was collected by filtration andcrystallized with a mixed solvent of methanol and toluene to obtain abifunctional methacrylate represented by the following formula (7).Molecular weight by mass analysis (ESI method): (M+H)⁺=385

Proton spectrum (400 MHz, solvent: deutero dimethyl sulfoxide, δ (ppm)):

TABLE 13 (7)

Assignment δ (ppm) Signal Number of protons 1 6.12 m 1 2 6.05 s 2 3 6.00m 2 4 5.85 d 1 5 5.67 s 2 6 4.05 t 4 7 3.55 m 1 8 3.30 to 3.22 m 4 92.96 m 2 10 1.88 s 6 11 0.96 d 3¹³C-NMR spectrum (100 MHz, solvent: deutero dimethyl sulfoxide, δ(ppm)):

TABLE 14 (7)

Number of carbon Assignment δ (ppm) Signal atoms 1 166.5 s 2 2 158.1 s 13 157.6 s 1 4 135.8 s 2 5 125.8 t 2 6 64.1 t 2 7 45.9 t 1 8 45.1 d 1 938.2 t 2 10 18.9 q 1 11 18.0 q 2

Example 1

In an argon-purged glove box, lithium hexafluorophosphate (LiPF₆) as anelectrolyte salt was dissolved in an ethylene carbonate/ethylmethylcarbonate mixed solvent (volume ratio: 1/2) such that the concentrationof the electrolyte salt became 1.2 moles/l, to prepare an electrolyticsolution. 3.0 g of the bifunctional methacrylate as prepared inReferential Example 1 was added to 97.0 g of this electrolytic solution,and the resulting mixture was stirred for dissolution at roomtemperature. 0.06 g of benzoyl peroxide was further added to themixture, which was then stirred for dissolution at room temperature.There was thus prepared a solution A.

Gelation Test and Heat Resistance Test of Gel

For the tests, in an argon-purged glove box, the solution A was chargedin a glass bottle, and the glass bottle was sealed and then placed in athermostatic chamber at 80° C. for 1 hour to polymerize the bifunctionalmethacrylate, thereby forming a crosslinked polymer. There was thusobtained a self-supporting gel electrolyte. This gel electrolyte wasallowed to stand in a sealed state within the thermostatic chamber at80° C. for 5 days. As a result, no separation of any liquid material wasobserved, and the resulting electrolyte maintained the gel state.

Preparation of Battery and Discharge Load Characteristic of Electrolyte

Each of a polyethylene resin-made porous film (thickness: 25 μm,porosity: 50%, mean pore size: 0.1 μm), a positive electrode made oflithium cobaltate (LiCoO₂) as an active material, and a negativeelectrode made of natural graphite as an active material was impregnatedwith the solution A. The resulting negative electrode, polyethyleneresin-made porous film and positive electrode were charged into abattery can (battery can for coin battery of 2016 size) also functioningas a pair of electrode cans in that order, to form a laminate comprisingnegative electrode/polyethylene resin-made porous film/positiveelectrode within the can. There was thus prepared an intermediate of acoin battery. Subsequently, the intermediate was placed in athermostatic chamber at 80° C. for 1 hour to polymerize the bifunctionalmethacrylate, thereby forming a crosslinked polymer and forming a gelelectrolyte. There was thus prepared a coin-type lithium-ion secondarybattery.

This battery was subjected to charge and discharge five times at a rateof 0.2 CmA. Thereafter, the battery was charged at a rate of 0.2 CmA andthen discharged at a rate of 2.0 CmA. Thus, the charge loadcharacteristic of the electrolyte was evaluated in a discharge capacityratio of 2.0 CmA/0.2 CmA. As a result, it was found to be 91%.

Example 2

A solution B was prepared in the same manner as in the preparation ofthe solution A of Example 1, except that the bifunctional methacrylateas prepared in Reference Example 2 was used in place of the bifunctionalmethacrylate as prepared in Reference Example 1.

Gelation Test and Heat Resistance Test of Gel

Using the solution B, a self-supporting gel electrolyte was obtained inthe same manner as in Example 1. In the same heat resistance test ofExample 1, this gel electrolyte was free from separation of any liquidmaterial and kept the gel state.

Preparation of Battery and Discharge Load Characteristic of Electrolyte

Using the solution B, a coin-type battery was prepared in the samemanner as in Example 1. The discharge load characteristic of theelectrolyte was evaluated under the same conditions as in Example 1. Asa result, it was found to be 90%.

Example 3

A solution C was prepared in the same manner as in the preparation ofthe solution A of Example 1, except that the bifunctional methacrylateas prepared in Reference Example 3 was used in place of the bifunctionalmethacrylate as prepared in Reference Example 1.

Gelation Test and Heat Resistance Test of Gel

Using the solution C, a self-supporting gel electrolyte was obtained inthe same manner as in Example 1. In the same heat resistance test ofExample 1, this gel electrolyte was free from separation of any liquidmaterial and kept the gel state.

Preparation of Battery and Discharge Load Characteristic of Electrolyte

Using the solution C, a coin-type battery was prepared in the samemanner as in Example 1. The discharge load characteristic of theelectrolyte was evaluated under the same conditions as in Example 1. Asa result, it was found to be 89%.

Example 4

A solution D was prepared in the same manner as in the preparation ofthe solution A of Example 1, except that the bifunctional methacrylateas prepared in Reference Example 4 was used in place of the bifunctionalmethacrylate as prepared in Reference Example 1.

Gelation Test and Heat Resistance Test of Gel

Using the solution D, a self-supporting gel electrolyte was obtained inthe same manner as in Example 1. In the same heat resistance test ofExample 1, this gel electrolyte was free from separation of any liquidmaterial and kept the gel state.

Preparation of Battery and Discharge Load Characteristic of Electrolyte

Using the solution D, a coin-type battery was prepared in the samemanner as in Example 1. The discharge load characteristic of theelectrolyte was evaluated under the same conditions as in Example 1. Asa result, it was found to be 92%.

Example 5

A solution E was prepared in the same manner as in the preparation ofthe solution A of Example 1, except that the bifunctional methacrylateas prepared in Reference Example 5 was used in place of the bifunctionalmethacrylate as prepared in Reference Example 1.

Gelation Test and Heat Resistance Test of Gel

Using the solution E, a self-supporting gel electrolyte was obtained inthe same manner as in Example 1. In the same heat resistance test ofExample 1, this gel electrolyte was free from separation of any liquidmaterial and kept the gel state.

Preparation of Battery and Discharge Load Characteristic of Electrolyte

Using the solution E, a coin-type battery was prepared in the samemanner as in Example 1. The discharge load characteristic of theelectrolyte was evaluated under the same conditions as in Example 1. Asa result, it was found to be 88%.

Example 6

A solution F was prepared in the same manner as in the preparation ofthe solution A of Example 1, except that the bifunctional methacrylateas prepared in Reference Example 6 was used in place of the bifunctionalmethacrylate as prepared in Reference Example 1.

Gelation Test and Heat Resistance Test of Gel

Using the solution F, a self-supporting gel electrolyte was obtained inthe same manner as in Example 1. In the same heat resistance test ofExample 1, this gel electrolyte was free from separation of any liquidmaterial and kept the gel state.

Preparation of Battery and Discharge Load Characteristic of Electrolyte

Using the solution F, a coin-type battery was prepared in the samemanner as in Example 1. The discharge load characteristic of theelectrolyte was evaluated under the same conditions as in Example 1. Asa result, it was found to be 93%.

Example 7

A solution G was prepared in the same manner as in the preparation ofthe solution A of Example 1, except that the bifunctional methacrylateas prepared in Reference Example 7 was used in place of the bifunctionalmethacrylate as prepared in Reference Example 1.

Gelation Test and Heat Resistance Test of Gel

Using the solution G, a self-supporting gel electrolyte was obtained inthe same manner as in Example 1. In the same heat resistance test ofExample 1, this gel electrolyte was free from separation of any liquidmaterial and kept the gel state.

Preparation of Battery and Discharge Load Characteristic of Electrolyte

Using the solution G, a coin-type battery was prepared in the samemanner as in Example 1. The discharge load characteristic of theelectrolyte was evaluated under the same conditions as in Example 1. Asa result, it was found to be 90%.

Comparative Example 1

In an argon-purged glove box, lithium hexafluorophosphate (LiPF₆) as anelectrolyte salt was dissolved in an ethylene carbonate/ethylmethylcarbonate mixed solvent (volume ratio: 1/2) such that the concentrationof the electrolyte salt became 1.2 moles/l, to prepare an electrolyticsolution. 3.0 g of polyethylene glycol dimethacrylate (NK ESTER 9G,manufactured by Shin-Nakamura Chemical Co., Ltd.) was added to 97.0 g ofthis electrolytic solution, and the resulting mixture was stirred fordissolution at room temperature. 0.06 g of 2,2′-azobisisobutyronitrilewas further added to the mixture. There was thus prepared a solution P.

Gelation Test and Heat Resistance Test of Gel

In an argon-purged glove box, the solution P was charged in a glassbottle, and the glass bottle was sealed and then placed in athermostatic chamber at 80° C. for 1 hour to polymerize the polyethyleneglycol dimethacrylate, thereby forming a polymer. However, the resultingelectrolyte was a flowable liquid. This liquid electrolyte was allowedto stand in a sealed state within the thermostatic chamber at 80° C. for5 days. As a result, it was discolored blackish brown, and a reductionof viscosity of the solution was observed.

Preparation of Battery and Discharge Load Characteristic of Electrolyte

Using the solution P, a coin-type battery was prepared in the samemanner as in Example 1. The discharge load characteristic of theelectrolyte was evaluated in a discharge capacity ratio of 2.0 CmA/0.2CmA in the same manner as in Example 1. As a result, it was found to be80%.

Comparative Example 2

In an argon-purged glove box, LiPF₆ as an electrolyte salt was dissolvedin an ethylene carbonate/ethylmethyl carbonate mixed solvent (volumeratio: 1/2) such that the concentration of the electrolyte salt became1.2 moles/l, to prepare an electrolytic solution. 20.0 g of polyethyleneglycol dimethacrylate (NK ESTER 9G; manufactured by Shin-NakamuraChemical Co., Ltd.) was added to 80.0 g of this electrolytic solution,and the resulting mixture was stirred for dissolution at roomtemperature. 0.4 g of 2,2′-azobisisobutyronitrile was further added tothe mixture. There was thus prepared a solution Q.

Gelation Test and Heat Resistance Test of Gel

In an argon-purged glove box, the solution Q was charged in a glassbottle, and the glass bottle was sealed and then placed in athermostatic chamber at 80° C. for 1 hour to obtain a self-supportinggel electrolyte. This gel electrolyte was allowed to stand in a sealedstate within the thermostatic chamber at 80° C. for 5 days. As a result,it was discolored blackish brown and changed to a flowable liquid.

Preparation of Battery and Discharge Load Characteristic of Electrolyte

Using the solution Q, a coin-type battery was prepared in the samemanner as in Example 1. The discharge load characteristic of theelectrolyte was evaluated under the same conditions as in Example 1. Asa result, it was found to be 40%.

Comparative Example 3

15 g of a vinylidene fluoride (VdF)/hexafluoropropylene (HFP) copolymerresin (KYNAR 2801, manufactured by Elf Atochem SA) and 30 g of dibutylphthalate were added to 200 g of acetone, and the resulting mixture wasmixed in a ball mill to prepare a homogeneous solution R. The solution Rwas cast in a uniform thickness on a tetrafluoroethylene resin sheet bya doctor blade, and the acetone was evaporated at 60° C. to form a filmhaving a thickness of 40 μm. This film was dipped in diethyl ether, andthe dibutyl phthalate was extracted to obtain a 40 μm-thick porous filmmade of a VdF/HFP copolymer resin.

In an argon-purged glove box, LiPF₆ as an electrolyte salt was dissolvedin-an ethylene carbonate/ethylmethyl carbonate mixed solvent (volumeratio: 1/2) such that the concentration of the electrolyte salt became1.2 moles/l, to prepare an electrolytic solution.

In an argon-purged glove box, the porous film was dipped in thiselectrolytic solution for one hour, thereby holding the electrolyticsolution on the porous film. There was thus prepared a gel electrolyte.Determining from an increase in weight before and after dipping theporous film in the electrolytic solution, a weight ratio of thecopolymer resin to the electrolytic solution of the gel electrolyte wasfound to be 30/70.

Heat Resistance Test of Gel

In an argon-purged glove box, this gel electrolyte was charged in aglass bottle, and the glass bottle was sealed and allowed to stand in asealed state within the thermostatic chamber at 80° C. for 5 days. As aresult, separation of the electrolytic solution was observed.

Preparation of Battery and Discharge Load Characteristic of Electrolyte

Using the foregoing gel electrolyte, a coin-type battery was prepared inthe same manner as in Example 1. The discharge load characteristic ofthe electrolyte was evaluated in a discharge capacity ratio of 2.0CmA/0.2 CmA in the same manner as in Example 1. As a result, it wasfound to be 73%.

As described above, the gel electrolyte according to the inventioncomprises as a matrix a crosslinked polymer prepared by polymerizing abifunctional (meth)acrylate represented by the foregoing general formulaas a gelling agent and has superior electrolytic characteristics.Moreover, according to the invention, the use of a small quantity of theforegoing gelling agent enables one to obtain a homogeneous gelelectrolyte having superior durability.

By using such a gel electrolyte, it is possible to obtain batteries orcapacitors that are free from a fear of liquid leakage and have highperformance and durability.

It should further be apparent to those skilled in the art that variouschanges in form and detail of the invention as shown and described abovemay be made. It is intended that such changes be included within thespirit and scope of the claims appended hereto.

This application is based on Japanese Patent Application No. 2002-86440filed Mar. 26, 2002, the disclosure of which is incorporated herein byreference in its entirety.

1. A process for producing a gel electrolyte which comprises heating orirradiating with actinic radiation a solution containing an electrolytesalt, a solvent for the electrolyte salt, and a bifunctional(meth)acrylate represented by the following formula (I):

wherein R represents a divalent group selected from: (a) an alkylenegroup, (b) a divalent group represented by the following general formula(II):

wherein R₂ represents an alkyl group, and R₃ and R₄ each independentlyrepresents an alkylene group, (c) a xylylene group, and (d) a divalentgroup represented by the following general formula (III):

R₁ represents a hydrogen atom or a methyl group, to polymerize thebifunctional (meth)acrylate, thereby forming a crosslinked polymer andforming a gel in which the electrolyte salt and the solvent are held ina matrix comprising the crosslinked polymer.
 2. A process for producinga gel electrolyte, which comprises heating or irradiating with actinicradiation a solution containing an electrolyte salt, a solvent for theelectrolyte salt, and a bifunctional (meth)acrylate represented by thefollowing formula (I):

wherein R represents a divalent organic group represented by thefollowing formula (III):

and R₁ represents a hydrogen atom or a methyl group, to polymerize thebifunctional (meth)acrylate, thereby forming a crosslinked polymer andforming a gel in which the electrolyte salt and the solvent are held ina matrix comprising the crosslinked polymer.